A variety of devices are used to determine the location of concealed underground objects. Determining the location of such concealed objects as underground gas, sewer and water pipes, power cables, and telephone and CATV cables or conduits is a necessary prerequisite to excavation and/or laying of new lines, pipes or cables. For simplicity, these underground objects are hereinafter referred to as underground "lines."
In some applications, an underground steerable boring tool is utilized to form an underground tunnel through which underground lines are subsequently routed. While using a steerable boring tool, it is important for an operator to trace or keep track of the relative location of the existing lines with respect to the boring tool, in order to avoid contacting the existing lines with the tool. In other applications, a trench is excavated and the lines are subsequently placed in the open trench. While excavating these trenches, it is equally important for an operator to know the location of any existing lines in order to avoid contacting them with the excavating equipment.
Special purpose electromagnetic signal detector systems, which are commonly called "locator systems," have been used for many years to locate and/or trace the path of the boring tool or concealed underground lines. Various types of locator systems exist, but receivers that detect electromagnetic signals radiating from either the underground lines or a small transmitter located within the boring tool are by far the most widely used. Such radiated signals are generally produced in two ways: (1) an alternating current signal from a transmitting above ground source is induced into a conductive line which generates an electromagnetic field around the line, or (2) a signal is radiated from a small transmitter either mounted inside a boring tool or positioned within a non-conductive line.
Generally two types of signal sources will induce a current in a conductive line which, in turn, will generate an electromagnetic field around the line: active signal sources and passive signal sources. An example of a passive signal source in the locating environment is the signal radiated from a broadcast station. When such a signal encounters an exposed portion of a buried conductive line, the signal induces a current in the underground portion of a line, which generates an electromagnetic field around the line. Such a source is called a passive signal source, because it requires no operator intervention to generate the electromagnetic field. The problem with a passive signal source is that the same signal may be induced into many different lines, which complicates the operator's task of distinguishing between different lines.
Conversely, an active signal source is intentionally utilized by an operator to generate an electromagnetic field directly associated with the object to be traced. For example, an operator may couple a signal having a known frequency of up to approximately 150 kHz to an underground cable, for the purpose of generating a distinct electromagnetic field around the cable. The presence of the distinct electromagnetic field allows the operator to locate the cable and distinguish it from other cables with an above-ground receiver. Another example of an active signal source is a small, underground transmitter which may be installed in a boring tool or passed through a non-conductive line. A distinct signal radiated from the underground transmitter allows the operator to trace either the path of the boring tool or that of the non-conductive line.
Both passive and active sources may be used with the present invention. Each has its advantages. A key advantage of an active signal source is the capability of coupling a distinctive frequency signal into one conductive line, and distinguishing that particular line from adjacent or nearby lines. Consequently, the conductive line of interest can be traced with less confusion or interference from adjacent lines. Since the frequency of a coupled signal can be controlled very precisely, a very narrow bandwidth may be used for greater selectivity in the receiver. Also, the use of a narrow bandwidth improves the signal to noise ratio and increases the sensitivity of the receiver. The use of a narrow bandwidth in a locator system can be especially important for locating conductive lines in the vicinity of a strong radio transmitter, where the airborne signals can mask a subsurface signal unless the airborne signals are filtered out by the receiver's selective, narrowband circuitry. Another advantage of an active source is that the locator's calculations of the position and depth of underground conductive lines are not effected by electromagnetic field distortions from multiple signal sources to the same degree as they are with a passive signal source.
A passive source has the advantage that no transmitter is necessary. A receiver is all that is required. Thus, operation is simpler if a passive source is employed.
Assuming an active source is used, the practical way to couple a signal to an underground conductive line is simply to attach a wire directly from the transmitter to the line. Such a technique is illustrated in U.S. Pat. No. 4,387,340 to Peterman. If this approach is not feasible, it is also possible to attach the transmitter wire to a toroidal clamp, which is placed around the circumference of the line in order to induce a current into the line. Alternatively, if the signal cannot be readily coupled directly to the line because, for example, the line is completely buried, the signal can be coupled indirectly into the line by using a coil located in the transmitter and passing an alternating current signal through the coil. The electromagnetic field from the coil in the above-ground transmitter radiates through the earth and induces current in the buried line. The signal induced in the line is then re-radiated from the line and detected with a locator receiver.
As shown in Peterman, an active signal source is commonly used when an operator desires to locate and trace a specific underground line that is near numerous other lines. A distinctive frequency signal is coupled from a locator transmitter to the line to be traced. In such an embodiment, the transmitter generates a signal at a specific frequency. The locator receiver is manually tuned to the frequency of the transmitted signal. Thus, the receiver operator can thereby distinguish the particular line which is radiating the transmitted signal from the other, nearby lines which are not radiating the transmitted signal.
More specifically, FIG. 1 illustrates a perspective view of a conventional above ground locator system utilizing an active signal source. Transmitter 10 is positioned on the surface of earth 15 above buried line 20, which is the concealed object to be traced. Transmitter output connector 12 is connected to a wire 18, which is in turn connected to conductive line 20. The connection of wire 18 to line 20 may be accomplished by directly attaching wire 18 to line 20, thus providing an electrical connection therebetween, or by connecting a toroidal clamp (not shown) to wire 18 and placing the clamp around line 20 to thereby induce the current. Thus, the output signal of transmitter 10, which is an AC continuous wave (CW) signal, is induced into line 20. Alternatively, if line 20 is buried to the extent that it is inaccessible by a wire attachment or toroidal clamp, the output signal is coupled to internal coil 11 in transmitter 10, which radiates an electromagnetic field corresponding to the CW signal. The radiated signal propagates through earth 15 and is induced into line 20. Consequently the output signal from transmitter 10, which is at a unique frequency, generates an electromagnetic field that radiates from line 20 with a field pattern 32 as illustrated in FIG. 2. Referring again to FIG. 1, receiver 30 is positioned on the surface 15 in the general vicinity of line 20 and manually tuned by an operator to the frequency of the transmitted signal. By sensing and processing the signal radiated from line 20, and using conventional locating techniques, the receiver operator locates the position of line 20 and traces the signal along the line's path.
FIG. 3 illustrates a perspective view of a conventional subsurface type active signal source locator transmitter which, in this case, is utilized for tracing the progress of a steerable, underground boring tool in order to avoid contacting existing lines with the tool. Referring to FIG. 3, transmitter 50 is placed within or closely behind boring tool 52. Transmitter 50 radiates a signal through earth 15 to above ground receiver 30. In response to location information provided by the receiver, an operator (not shown) of boring tool 52 rotates drill string 54 about its boring axis to control the direction of boring tool 52.
In another conventional application, it is sometimes required to detect and trace the paths of plastic or concrete underground pipes. Since these lines are non-conductive, there is no way to trace them by inducing an alternating current signal in them and detecting the radiated electromagnetic field. Consequently, a small subsurface transmitter is inserted into the plastic or concrete line, and the electromagnetic field radiated from the transmitter is detected by an above ground receiver and traced by an operator along the path of the line. Referring to FIG. 4, subsurface transmitter 60, which utilizes an active signal source, includes coil 62 which is wrapped around ferromagnetic rod 64. Coil 62 is energized with a closely controlled signal frequency by oscillator 66. Oscillator 66 is powered by an internal battery (not shown). Subsurface transmitter 60 is attached to rod 68 and pushed down the length of plastic or concrete line 65. Receiver 30 detects and processes the electromagnetic field radiated by transmitter 60. Consequently, a locator operator can trace the position of subsurface transmitter 60 as it is routed through line 65.
An example of a passive source is shown in FIG. 5. Broadcast station 61 transmits electromagnetic waves 62 which are coupled into line 59 through an exposed portion of the line 63. These electromagnetic waves are re-radiated from line 59 and detected by receiver 60. Thus, the location of underground line 59 may be ascertained.
Conventional locator systems are relatively inflexible and inefficient from an operational standpoint, because of their use of analog filters. These filters require factory tuning and are quite susceptible to damage in the field. Furthermore, the selectivity of the analog filters is quite limited, and this adversely affects their accuracy and performance.